A conventional ultrasound image is composed of multiple image scan lines. A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line's resolution is a result of the directivity of the associated transmit and receive beam pair.
The outputs of the beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as an image of the anatomy being scanned.
Referring to FIG. 1, a conventional ultrasonic imaging system comprises a transducer array 10 consisting of a plurality of separately driven transducer elements 12, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter 14. The ultrasonic energy reflected back to transducer array 10 from the object under study is converted to an electrical signal by each receiving transducer element 12 and applied separately to a receiver 16 through a set of transmit/receive (T/R) switches 18. The T/R switches 18 are typically diodes which protect the receive electronics from the high voltages generated by the transmit electronics. The transmit signal causes the diodes to shut off or limit the signal to the receiver. Transmitter 14 and receiver 16 are operated under control of a master controller 20 responsive to commands by a human operator. A complete scan is performed by acquiring a series of echoes in which transmitter 14 is gated ON momentarily to energize each transducer element 12, and the subsequent echo signals produced by each transducer element 12 are applied to receiver 16. A channel may begin reception while another channel is still transmitting. The receiver 16 combines the separate echo signals from each transducer element to produce a single echo signal which is used to produce a line in an image on a display monitor 22.
Under the direction of master controller 20, the transmitter 14 drives transducer array 10 such that the ultrasonic energy is transmitted as a directed focused beam. To accomplish this, respective time delays are imparted to a multiplicity of pulsers 24 by a transmit beamformer 26. The master controller 20 determines the conditions under which the acoustic pulses will be transmitted. With this information, the transmit beamformer 26 will determine the timing and the amplitudes of each of the transmit pulses to be generated by the pulsers 24. The amplitudes of each transmit pulse will be determined by a apodization generation circuit 36 which applies respective apodization weighting factors to the pulsers. For example, the apodization generation circuit could comprise a high-voltage controller which sets the power supply voltage to each pulser. The pulsers 24 in turn send the transmit pulses to each of the elements 12 of the transducer array 10 via the T/R switches 18, which protect the time-gain control (TGC) amplifiers from the high voltages which may exist at the transducer array. The apodization weightings are generated within the apodization generation block 36, which could further comprise a set of digital-to-analog converters which take the weighting data from the transmit beamformer 26 and apply it to the pulsers 24 via the aforementioned high-voltage controllers.
The echo signals produced by each burst of ultrasonic energy reflect from objects located at successive ranges along each ultrasonic beam. The echo signals are sensed separately by each transducer element 12 and a sample of the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range.
Due to the differences in the propagation paths between a reflecting point and each transducer element 12, the echo signals will not be detected simultaneously and their amplitudes will not be equal. Receiver 16 amplifies the separate echo signals via a respective TGC amplifier 28 in each receive channel. The amount of amplification provided by each TGC amplifier is controlled through a respective control line (not shown) that is driven by a TGC circuit (not shown), the latter being set by hand operation of a respective one of a multiplicity of potentiometers. The amplified echo signals are then fed to the receive beamformer 30. Each receiver channel of the receive beamformer is connected to a respective one of the transducer elements 12 by a respective TGC amplifier 28.
Under the direction of master controller 20, the receive beamformer 30 tracks the directions of the transmitted beam, sampling the echo signals at a succession of ranges along the beam. The receive beamformer 30 imparts the proper time delay to each amplified echo signal. The receive focus time delays are computed in real-time using specialized hardware or read from a look-up table. The receive channels also have circuitry for filtering the received pulses. The time-delayed receive signals are then summed to provide an echo signal which accurately indicates the total ultrasonic energy reflected from a point located at a particular range along the ultrasonic beam. The summed receive signals are output to a signal processor or detector 32. The detector 32 converts the summed received signals to display data. Preferably detector 32 is an envelope detector. In the B-mode (grey-scale), the envelope of the signal is subjected to additional processing (commonly referred to as "post-processing"), such as edge enhancement and logarithmic compression.
The scan converter 34 receives the display data from the post-processor (not shown) and converts the data into the desired image for display. In particular, the scan converter 34 converts the acoustic image data from polar coordinate (R--.theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data at the video rate. This scan-converted acoustic data is then output for display on display monitor 22, which images the time-varying amplitude of the envelope of the signal as a grey scale. A respective scan line is displayed for each separate transmitted beam.
Some conventional ultrasound imaging systems are capable of operation in a continuous wave mode. A continuous wave transmission has application in ultrasound imaging of the heart. When operated in a continuous wave mode, means must be provided for subtracting the carrier signal from the receive signal. Ideally about 20 dB of carrier rejection should be provided. The carrier rejection means preferably occupy very little space, use very little power and are inexpensive in order to provide a commercially viable ultrasound imaging system having a continuous wave mode.